1. Field of the Invention
The present invention relates to a lightweight battery package for sealing an electrode assembly.
2. Description of the Prior Art
Batteries are utilized as main power supplies or backup sources in variety of devices. Recently, with an astonishing advance in portable electric devices, there is a high level interest in high-performance rechargeable batteries, such as lithium ion batteries.
A conventional lithium ion battery is provided as a sealed battery in a metal package, in which a cathode, an anode and an electronically insulating and electrolyte-holding separator interposed between the electrodes are incorporated in a shape of a cylinder. However, since the package is made of metal, the battery is heavy and difficult to reduce its thickness. Accordingly, with the development of portable electric devices, there exists a need in the art for a battery which has light weight and thin body.
One of the solution of the problem is disclosed in the Japanese laid-open patent publication No.H10-172606, in which a laminate sheet, a polymer sheet having metal layer such as Al layer evaporated thereon, is used as a package instead of the conventional metal package.
FIGS. 6A and 6B shows a structure of such battery using the laminate-sheet package. An electrode assembly 2 including a cathode, an anode and a separator, which are unified by adhesion, is enveloped with electrolyte in the package 7 which is formed with two laminate sheets 4 and 6. The laminate sheets 4 and 6 are mainly composed of polymer sheet in order to reduce its weight and have metal layer evaporated thereon. The metal layer helps the permeable polymer sheet to shut off moisture. The metal layer is made of lightweight Al and has thickness of about 10 to 15 xcexcm.
The inner surfaces of the laminate sheet 4 and 6 are made of heat-adhesive polymer. The laminate sheets 4 and 6 are heat-sealed together at the periphery of the electrode assembly 2 to seal the package 7. The heat-sealed portion 8 protrudes outwardly from the side faces of the battery. In order to prevent the water permeation into the package for long periods, the heat-sealed portion 8 must be formed wide, the width of which is generally several to 10 mm.
The lead terminals 12 for electrically accessing the electrode assembly 2 from outside of the package are drawn out through the seal portion 8. Thus constructed battery is lighter and thinner than the battery using the metal package.
However, such constructed light and thin battery has some drawbacks as described below. Since the sealed portion 8 of the laminate sheets 4 and 6 has a flange-like shape protruding outwardly from the side face of the battery, a projection area of the battery is larger than that of the metal-package battery. This reduces its volumetric energy density. On the other hand, the width of the seal portion 8 affects the sealing ability of the battery. If the width of the seal portion 8 is narrow, it is impossible to prevent moisture permeation from air for a long duration of the battery operation. Particularly, the lithium ion secondary battery may generate HF by the chemical reaction of the electrolyte with the permeated water, and the generated HF causes peeling of the seal. For keeping high reliability, it is preferable that the width of the seal portion is as wide as possible. In general, the width of the seal portion must be greater than several mm in minimum. Therefore, there exists a trade-off between the volume energy density and the reliability of the battery.
It is accordingly an object of the present invention to provide a battery package which can reduce weight and thickness of the battery and has small projection area and high reliability.
According to the present invention, the battery package comprises laminate sheets adhered each other along their peripheral to form a container portion for receiving an electrode assembly and a seal portion surrounding said container portion and protruding outwardly from side faces of said container portion, said seal portion having enough width to maintain said container portion free from moisture for long periods of time,
wherein said laminate sheets include a heat-adhesive polymer layer and a metal layer which stops moisture and provides a shape-maintaining ability to said laminate sheets,
and wherein said seal portion is folded or curled to reduce a projection area of said battery package.
This battery package improves the volumetric energy density of the battery by reducing its projection area, while maintaining the reliability of the battery with wide seal portion.
The laminate sheets preferably maintain their shape, when folded or curled, without breaking the metal layer therein. For this purpose, the thickness of the metal layer is preferably in a range of 30 to 100 xcexcm and larger than half of the thickness of the polymer layer. This prevents an occurrence of cracks in the metal layer when the seal portion is folded or curled, and thus improves the reliability of the battery.
The metal sheet is preferably made of Al or Al alloy to reduce the weight of the package.